Recent advances in the fabrication of ultratransparent materials have demonstrated that fibers are a promising transmission medium for optical communication systems. These liqht transmitting fibers, which are referred to as optical waveguides generally consist of a transparent core surrounded by a layer of transparent cladding material having a refractive index which is lower than that of the core. A low loss optical waveguide and a method of making the same are disclosed in U.S. Pat. No. 3,659,915. The waveguide described in that patent comprises a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding.
If such fibers are to be used in optical communication systems, means must be provided for quickly and conveniently connecting sections of fiber together in the course of their use. Such means must cause light to be coupled from one fiber to another with little loss of signal, i.e., with low insertion loss. When two optical waveguides are joined end-to-end, insertion loss can result from center-to-center mismatch, angular misalignment of the fiber axes, fiber-to-fiber separation, rough end finish and Fresnel reflections. For example, consider the insertion loss which may occur when two fused silica optical waveguides of the type disclosed in the aforementioned U.S. Pat. No. 3,659,915 are joined end-to-end. For this illustration, it is assumed that the fibers have a diameter of 5 mils and a cladding thickness of 1 mil. If misregistration of the centers of the two fibers is the only loss mechanism, a loss of about 1 db occurs if the centers thereof are separated by 0.5 mil. For reasonably low insertion losses to be achieved, the centers of the fiber endfaces must therefore be registered within 0.5 mil. Furthermore, due to the low numerical aperture of present optical waveguides, the fiber axes must be substantially aligned, i.e., they must be aligned to within 3.degree. to keep insertion losses less than about 1 db, assuming no other loss mechanisms are operative. Since light diverges from the fiber axis aas it radiates from a fiber, some light is lost if the endfaces of the two fibers are separated; therefore, the endfaces of the fibers should be maintained in virtual contact. Fiber-to-fiber separation also implies an insertion loss due to Fresnel reflections at the two glass-air interfaces. For the aforementioned fused silica optical waveguides, this amounts to 0.15 db per interface. This loss, however, can be substantially eliminated by disposing between the fiber endfaces a layer of index matching such as an oil or other fluid having a refractive index of about 1.5.
Minimizing these insertion losses becomes especially difficult when bundles of fibers are to be connected. The axes of all fibers in both bundles should be substantially parallel, the endfaces of the fibers in one bundle should be virtually touching the endfaces of the fibers in the other bundle, and index matching fluid should be disposed between the ends of the fibers in the two bundles. Finally, the endface of a fiber in one bundle should be substantially centered with respect to that of a corresponding fiber in the other bundle. A particularly important feature of the present invention concerns this latter requirement.